Ciap. XI.] EXTINCTION. 95 



naturally lead them to this conclusion. On the contrary, 

 we have every reason to believe, from the study of the 

 tertiary formations, that species and groups of species 

 gradually disappear, one after another, first from one spot, 

 then from another, and finally from the world. In some 

 few cases however, as by the breaking of an isthmus 

 and the consequent irruption of a multitude of new 

 inhabitants into an adjoining sea, or by the final subsi- 

 dence of an island, the process of extinction may have 

 been rapid. Both single species and whole groups of 

 species last for very unequal periods ; some groups, 

 as we have seen, have endured from the earliest known 

 dawn of life to the present day ; some have disappeared 

 before the close of the palaeozoic period. No fixed law 

 seems to determine the length of time during which 

 any single species or any single genus endures. There 

 is reason to believe that the extinction of a whole group 

 of species is generally a slower process than their pro- 

 duction : if their appearance and disappearance be re- 

 presented, as before, by a vertical line of varying thick- 

 ness the line is found to taper more gradually at its upper 

 end, which marks the progress of extermination, than 

 at its lower end, which marks the first appearance and 

 the early increase in number of the species. In some 

 cases, however, the extermination of whole groups, as 

 of ammonites, towards the close of the secondary period, 

 has been wonderfully sudden. 



The extinction of species has been involved in the 

 most gratuitous mystery. Some authors have even sup- 

 posed that, as the individual has a definite length of 

 life, so have species a definite duration. No one can 

 have marvelled more than I have done at the extinction 

 of species. When I found in La Plata the tooth of a 



